In oil and gas wells, the wellbore may be open or may be clad with a well casing. Visual inspection of the wellbore is important to check the integrity of the wellbore, and to investigate any downhole problems that may delay or prevent use of the well. For example, it is important to regularly inspect the casings for corrosion and wear.
Although visual inspection of the wellbore is important, the conditions typically found in a wellbore tend to hinder the ability to use many camera systems. Wellbores can have diameters in the range 10 centimeters to 1 meter and can reach depths of hundreds or thousands of meters. In order to inspect these bores, therefore, it is not only necessary to provide a camera system that can operate at these depths, but also to provide the lighting required to be able to capture still images or video in this confined environment.
Furthermore, any camera system must be able to withstand the pressures and temperatures encountered at depth in a borehole. Pressures at these depths can be very large and can reach around 150 MPa, and in addition, temperatures may exceed 100.degree. C.
Typically, downhole camera systems comprise a camera and light source contained in a protective steel sheath. These camera systems are lowered into the wellbore on an electrical cable or a shaft, with the images from the camera being relayed back to the surface where they are displayed and recorded.
The confined environment of the wellbore causes problems in designing a camera and lighting arrangement that is small enough while still delivering high enough light levels to capture the required images.
Several camera systems use a backlight system in which the light source is mounted at a distance behind the camera. The light is then directed into the field of view of the camera by means of a reflector mounted adjacent to the camera. However, this approach is less successful in narrower passageways as the size of the camera becomes too large compared to the diameter of the bore to allow sufficient light to be reflected.
It is also known to provide an array of light emitting diodes (LEDs) as the light source due to their relatively low power consumption and small size. These LEDs are typically mounted around the outside of the camera approximately level with the camera lens. The LEDs therefore directly illuminate the field of view of the camera.
In order to protect the camera and the light source from the harsh environment of the wellbore, a cover or window is typically placed over the distal end of the camera system. Any light emitted from the light source, therefore, must pass through this window before it illuminates the wellbore.
This has a disadvantage, however, because some of the light that travels through the window is internally reflected and does not pass through the window.
Furthermore, some of the internally reflected light is directed back towards the lens of the camera, leading to poor images.
It is an object of the present invention to provide an improved window for a subsea camera system that overcomes these problems.